Cellular communication systems are being developed and improved for machine type communication (MTC), communication characterized by lower demands on data rates than for example mobile broadband, but with higher requirements on, for example, low cost device design, better coverage, and an ability to operate for years on batteries without charging or replacing the batteries. In the 3GPP GERAN specification group, cellular communication systems are being improved and developed in the feasibility study named “Cellular System Support for Ultra Low Complexity and Low Throughput Internet of Things.” GSM is being evolved, and new “clean slate” systems (systems not based on current cellular systems) are being developed.
One “clean slate” solution, called narrowband machine-to-machine (NB M2M), is a narrowband system with a carrier bandwidth of 200 kHz that targets improved coverage compared to GSM systems, long battery life, and low complexity communication design. One intention with this solution is to deploy it in spectrum that is currently used for GSM, by reducing the bandwidth used by GSM and deploying NB M2M in the spectrum that becomes available. Another intention is to reuse existing GSM sites for the deployment of NB M2M.
In cellular communication systems, devices use a cell search procedure (or synchronization procedure) to understand which cell(s) to connect to. Some of the functions of a cell search procedure include detecting a suitable cell to camp on, and for that cell, obtaining the symbol and frame timing and synchronizing to the carrier frequency. When synchronizing to the carrier frequency, the mobile station needs to correct any erroneous frequency offsets that are present, and perform symbol timing alignment with the frame structure from the base station.
When the device wakes up from deep sleep, for example from being in a power saving state, the frequency offset is to a large extent due to device clock inaccuracy (often assumed to be up to 20 ppm). The clock inaccuracy appears mainly as a frequency offset of the received signal, a continuous rotation of the received samples. For a system operating with a carrier frequency of 900 MHz, the maximum frequency offset is 18 kHz (corresponding to 20 ppm inaccuracy). This offset needs to be estimated and corrected for.
The cell search procedure for NB M2M is described in GP-140864, “NB M2M—Cell Search Mechanism,” and GP-140861, “NB M2M—Frame Index Indication Design.” A physical channel named Physical Broadcast Synchronization Channel (PBSCH) is dedicated to carrying the synchronization signals, along with the broadcast system information. A separate downlink physical channel per base station is reserved for PBSCH, while the data channels are multiplexed by frequency division multiplexing (FDM). In addition, the PBSCH operates with a reuse factor of 1, implying that the PBSCH of neighboring cells are completely overlapped in the frequency domain. This has the advantage of a reduction in search complexity, but also results in interference from all the other cells using the PBSCH. As described in GP-140864, frame timing estimation and frequency offset correction is performed using two different sequences:                (a) Primary Synchronization Sequence (PSS): The PSS is used to determine the frame timing alignment, along with a coarse estimation of the frequency offset.        (b) Secondary Synchronization Sequence (SSS): The SSS is used to obtain a finer estimate of the frequency offset.        
FIG. 1 illustrates a frame structure for PBSCH. More particularly, FIG. 1 illustrates a number of frames 5a, 5b, 5c, 5d (corresponding to the 0th Frame, 1st Frame, 2nd Frame, and 63rd Frame, respectively). Every frame, such as 2nd Frame 5c, consists of 960 symbols. In the example frame structure shown in FIG. 1, 2nd Frame 5c includes PSS 10, SSS 15, and Frame Index Indication Sequence (FIIS)+Broadcast Information Block (BIB) 20. In 2nd Frame 5c, 256 symbols are dedicated to PSS 10, 257 symbols are dedicated for SSS 15, and 447 symbols are dedicated to FIIS+BIB 20 (of the 447 symbols, 127 symbols are dedicated for FIIS, and the remaining 320 symbols are for carrying the broadcast information in a BIB.
After switching on, an MTC device first needs to search for a signal in a viable frequency band. Signal detection is performed on the basis of comparing the amplitude of the peak from a correlation based detector with a pre-determined threshold. This is achieved by correlating the received signal with a known sequence, or a set of known sequences. Timing offset estimation and frequency offset estimation can then be performed as described in GP-140864.
A problem with this approach arises from the fact that the maximum frequency offset can be +/−18 kHz, whereas the signal bandwidth is only 12 kHz. Since the maximum frequency offset is larger than the signal bandwidth, using the existing approach for detecting the frequency offset described in GP-140864 leads to aliasing. With existing frequency offset detectors that have a sampling rate corresponding to the signal bandwidth of 12 kHz, or those that perform linear operations such as correlation of the transmitted signal with the received signal in time or frequency domain, one can only detect frequency offsets in the range [−6,6] kHz, and any frequency offset outside this range will be aliased and incorrectly detected as being in the range [−6,6] kHz.